1. Field of Invention
The present invention relates to the field of motors, and more particularly but not by way of limitation, to a concatenated motor assembly that includes a first stator inducing a current in a rotor adjacent the first stator for use in powering a second stator.
2. Discussion
A variety of systems are used to bring fluids from below ground to the surface in a well when the pressure is insufficient or it is beneficial for other reasons. One common method involves using a pumping system to draw fluids from the producing formation(s) to the surface for collection and processing. In one class of pumping systems, a submersible pumping unit is immersed in the well-bore fluids and driven to force fluids through production tubing to the earth's surface. Such pumping systems typically include an electric submersible motor (ESM), a submersible production pump with sealing portions to protect the motor from well-bore fluids, a gearbox, and a variety of other controls such as a variable speed drive (VSD).
In many pumping systems, centrifugal pumps are used but centrifugal pumps are not adequate in a number of circumstances. In particular, centrifugal pumps are typically inefficient at lower pump speeds. Alternatives to centrifugal pumping systems include positive displacement pumping systems, such as a progressive cavity pumping systems (PCS). During the start-up phase of the pumping system a higher torque is needed from a motor portion of the pumping system to drive the progressive pump portion of the pumping system. In order to provide the higher torque required at start-up, and to speed match the motor to the operating range of the progressive cavity pump portion of the system, the progressive cavity pumping system usually includes a gear reducer for increasing motor output torque and speed matching.
Typically, such gear reducers are positioned within the well-bore and thus are size constrained. Also, such gear reducers operate at speeds determined by a fixed ratio of the output speed of the motor, so motors of the progressive cavity pumping system generally need to be coupled with a variable speed driver to effect operation of the prior art progressive cavity pumping system over a range of speeds. Even when a variable speed drive is used, the gear reducers limit the range of speeds for operating the progressive cavity pump portion of a progressive pumping system, typically making higher production rates unavailable. Thus prior art progressive cavity pumping systems ordinarily fail to afford the flexibility necessary to pump fluids at both low and high flow rates.
Within a typical prior art progressive cavity pumping system, a motor coupled to a variable speed drive exhibits decreasing torque in response to an input from the variable speed drive for a lower rotational speed and show significant decreases in available torque for current supplied at frequencies below 30 Hertz. Additionally, the maximum torque transfer of a gearbox assembly within a typical prior art progressive cavity pumping system is limited by the gearbox size, specifically an available diameter for the gears of the gearbox; thus a well-bore diameter often limits the available horsepower of a typical prior art progressive cavity pumping system. Within a typical well-bore, the available horsepower of most progressive cavity pumping systems equipped with a gearbox and operating under a variable speed drive is limited to about 80 horsepower. Furthermore, the inclusion of a gearbox and a variable speed drive in a prior art progressive cavity pumping system add significantly to the cost of the system.
Variable speed drives (VSD) are often used in conjunction with a gearbox within a prior art progressive cavity pumping system to achieve a wider operating speed range but an alternative method is to use the VSD directly with an ESM to run the motor in a controlled low speed operation. However, a prior art progressive cavity pumping system with a variable speed drive coupled directly to a motor of the system typically has a limiting starting torque, which often proves to be insufficient for a system utilizing a progressive cavity pump that requires a starting torque of nearly 145% of the running torque of the system. Also, a prior art progressive cavity pumping system configured with a variable speed drive coupled directly to an electric submersible motor is horsepower limited and non-applicable to a number of submersible applications.
Therefore, challenges remain and a need persists for a cost competitive, progressive cavity pumping system compliant with high torque start-up demands placed on the system, while providing improved reliability for steady state operation of the pumping system.